Japan approved its 2011 defense white paper earlier in the day, using the same language since 2005 to refer to Dokdo as Japanese territory. The East Sea islets have been a source of ongoing tension between the neighbors as South Korea flatly dismisses Japan's claims as nonsense, saying it regained control over all of its territory, including Dokdo, at the end of Tokyo's 1910-45 colonial rule.

Tourists abound on South Korea's easternmost islets of Dokdo in the East Sea on Aug. 1. (Yonhap)

The move comes one day after three Japanese lawmakers of the conservative Liberal Democratic Party were forced to return home after being denied entry at a Seoul airport over their plan to visit Ulleung Island, located 90 kilometers west of Dokdo. The trip was seen as an attempt to bolster Japan's claims to South Korea's easternmost territory, and Seoul justified its response based on its immigration laws, which allow an entry ban for people who could harm the country's interests or the public's safety.

On Tuesday, the foreign ministry summoned Nobukatsu Kanehara, a diplomatic minister at the Japanese embassy here, to express regret over the defense paper's claims to Dokdo. In previous years, the ministry has summoned the Japanese embassy's councilor, who has lower rank than a minister.

Kanehara's visit came only a day after he called the ministry to protest Seoul's entry ban on the Japanese lawmakers.

The ministry's spokesman also plans to issue a statement later in the day to protest Tokyo's latest move, instead of a lower-ranking ministry official who has been in charge of it in the past.

Nobukatsu Kanehara, a diplomatic minister at the Japanese embassy to Seoul, enters South Korea's foreign ministry after being summoned over Japan's claims to Dokdo in its 2011 defense white paper. (Yonhap)

"The government's consistent and firm position is that Dokdo is our territory in terms of history, geography and international law," a ministry official told reporters on the condition of anonymity. "From this perspective, I deeply regret the fact that the Japanese government has again included unreasonable claims to Dokdo in this year's defense white paper, which are unhelpful to our bilateral relations."

This year's defense paper repeats the same phrase that has been included since 2005, claiming that "the territorial issue of the Northern Territories and Takeshima, which are proper to Japan, remains in an unresolved state." The Northern Territories refer to islands off Japan claimed by both Tokyo and Moscow, while Takeshima is the Japanese name for Dokdo.

The defense ministry here also expressed "deep regrets" over Tokyo's latest claim to Dokdo. In a statement, the ministry said it "sternly protests" the publication of the latest defense white paper and demanded the Japanese government immediately withdraw its territorial claim.

"We urge the Japanese ministry to realize it can't expect future-oriented development of our bilateral military relations unless Japan relinquishes its claim to Dokdo," the statement read. "We will firmly deal with any attempts to damage the sovereignty of Dokdo."

To lodge complaint, the defense ministry summoned Hiroshi Kimura, a military attache at the Japanese embassy here. Kimura was also summoned last year when the defense paper was released.

South Korea has stationed its Coast Guard on Dokdo since 1954 as a symbol of its ownership. Meanwhile, Japan continues to lay claim to the islets in its school textbooks and official government documents, each time reigniting deep resentment among South Koreans over Tokyo's colonial rule.

The latest diplomatic spat came early last month when Japan imposed a one-month ban on the use of Korean Air flights by its diplomats in retaliation for the airline's June 16 test flight of its first Airbus A380 plane over Dokdo. South Korea demanded an immediate withdrawal of the measure, but Tokyo refused to do so.